For several decades, fiber optics have been used for communication. Specifically, fiber optics are used for data transmission and other telecommunication applications. Despite the enormous information carrying capacity of fiber, as compared to conventional copper cable, the high cost of installing fiber optics presents a barrier to full implementation of fiber optics, particular as the “last mile”, from the central office to residences and businesses.
One method of increasing carrying capacity without incurring additional installation costs has been to multiplex multiple signals onto a single fiber using various methods, such as time division multiplexing, where two or more different signals are carried over the same fiber, each sharing a portion of time. Another, more preferred multiplexing method is wavelength division multiplexing (WDM), where two or more different wavelengths of light are simultaneously carried over a common fiber.
Until recently, typical fibers used for communications applications had preferred wavelength bands centered at 850 nm, 1300 nm, and 1550 nm, wherein each band typically had a useful bandwidth of approximately 10 to 40 nm depending on the application. Transmission within these bands was preferred by systems designers because of low optical attenuation. Recent advances in fiber design now provides fiber that have low attenuation over a very broad transmission range, from 1300–1620 nm.
Wavelength division multiplexing can separate a fiber's bandwidth into multiple channels. Dividing bandwidth into multiple discreet channels, such as 4, 8, 16, 40, or even as many as 160 channels, through a technique referred to as dense channel wavelength division multiplexing (DWDM), is a relatively lower cost method of substantially increasing telecommunication capacity, using existing fiber optic transmission lines. Techniques and devices are required, however, for multiplexing the different discreet carrier wavelengths. That is, the individual optical signals must be combined onto a common fiber-optic line or other optical waveguide and then later separated again into the individual signals or channels at the opposite end or other point along the fiber-optic cable. Thus, the ability to effectively combine and then separate individual wavelengths (or wavelength sub-ranges) from a broad spectral source is of growing importance to the fiber-optic telecommunications field and other fields employing optical instruments.
Optical multiplexers are known for use in spectroscopic analysis equipment and for the combination or separation of optical signals in wavelength division multiplexed fiber-optic telecommunications systems. Known devices for this purpose have employed, for example, diffraction gratings, prisms and various types of fixed or tunable filters.
Approaches for selectively removing or tapping a channel, i.e., selective wavelengths, from a main trunk line carrying multiple channels, i.e., carrying optical signals on a plurality of wavelengths or wavelength sub-ranges, is suggested, for example, in U.S. Pat. No. 4,768,849 to Hicks, Jr. Hicks, shows filter taps, as well as the use of gangs of individual filter taps, each employing high performance, multi-cavity dielectric pass-band filters and lenses for sequentially removing a series of wavelength sub-ranges or channels from a main trunk line. The filter tap of Hicks, returns a multi-channel signal to the main trunk line as it passes the desired channel to a branch line. One known demux is disclosed in Pan et al., U.S. Pat. No. 5,652,814, FIG. 25. In Pan et al., the WDM input signal is cascaded through individual filter assemblies, consisting of a fiber collamator, thin film filter, and a fiber focusing lens. Each filter is set for a given wavelength. However, aligning the fibers for each wavelength is costly and errors in the alignment contribute significantly to the system losses. Further, FIG. 13 of Pan et al. teaches the use of a dual fiber collimator, thin film filter, and a dual fiber focusing lens to selectively DROP and ADD a single wavelength or range of wavelengths. As discussed above, aligning the collimators is expensive. Other optical multiplexing devices eliminate many of the fiber to lens alignments.
In U.S. Pat. No. 4,244,045 to Nosu et al, for multiplexing or demultiplexing a multi-channel optical signal. A row of individual optical filters are glued side-by-side onto the surface of an optical substrate, and a second row is similarly glued to the opposite surface of the substrate. Each individual filter transmits a different channel, that is, a preselected wavelength(s), and reflects other wavelengths. A multi-channel optical beam from a trunk line enters the optical substrate at an angle and passes through the substrate from filter to filter in a zig-zag fashion. Each filter transmits its preselected wavelength(s) and reflects the remainder of the beam on to the next filter. Each filter element is sandwiched between glass plates, and a prism element is positioned between each filter sandwich and a corresponding collimator positioned behind the filter sandwich to receive the transmitted wavelength(s). Nosu et al teaches the use of refractive index matching. The lenses, filters, optical substrate, etc. all have the same refractive index and are in surface-to-surface contact with one another, such that the light beam does not pass through air. This approach by Nosu et al involves the use of prisms as an optical bridge between the filter element and the collimators at each channel outlet. This elaborate design approach adds considerable cost and assembly complexity to multiplexing devices of the type shown in Nosu et al. The approach of Scobey, et. al, in U.S. Pat. No. 5,859,717, is similar to Nosu et al., except the zig-zag pattern is through air, not glass. A single spacer block with a hole is used to mount the individual filter. The block is dense and stable with low sensitivity to changes in the temperature and ambient humidity. Xu, in U.S. Pat. No. 6,118,912, also uses a zig-zag pattern between filters in air, but Xu tilts the individual filters to adjust the center bandpass of each of the wavelengths. Thin film multiplexing devices are economical for low channel count systems and have a desirable flat-topped pass bands. Those skilled in the art will recognize that these multiplexing devices can also be employed in reverse to multiplex optical signals from individual channels onto a multi-channel optical signal.
Polarization dependent loss (PDL) is also a problem in WDM system because the polarization of the light drifts as it propagates through the fiber and furthermore this drift changes over time. Thus, if there is PDL in any component, the drifting polarization will change the signal level, which may degraded the system operation.
Other multiplexer devices may be employed to add or drop channels in WDM systems. These systems are commonly known as optical add/drop multiplexers, or OADM. Another OADM, disclosed by Mizrahi U.S. Pat. No. 6,185,023, employs a fiber Bragg grating to a demux and mux signals in a WDM system. This method requires optical circulators and multiple components.
However, the multi channel OADM designs discussed above are not programmable by the end user. That is, each multiplexers is designed and manufactured to mux (add) specific channels by the factory; or when used in reverse each multiplexers is also designed and manufactured to demux (drop) specific channels by the factory. This limitation mandates that the optical system's parameters be fixed before installation. Changes are not possible without replacing the fixed optical multiplexers with different designed multiplexers. This is expensive.
One known programmable OADM is discussed in Boisset et al, International Publication No. WO01/13151. In Boisset et al., the desired add/drop channel is programmed by translating a segmented filter. To achieve this translation however, a large mechanical mechanism is employed. A further limitation to Boisset et al. is that only a single channel may be added or dropped per device. Designers may employ multiple devices, deployed in series, and programed as necessary to add/drop the correct channel, however this approach requires multiple devices and has multiple points of failure. Furthermore, the size of such a device would be overly large and therefore not practical for many applications where space is limited.
Two other programmable OADMs are disclosed by Tomlinson, U.S. Pat. No. 5,960,133, and Aksyuk, et al, U.S. Pat. No. 6,204,946, both use bulk optics and gratings to demultiplex and multiplex WDM input and output signal. The grating-based systems are large, inefficient and tend to have sharply peaked pass bands, rather then the desired flat-topped pass bands.
It is an object of the present invention to provide improved optical multiplexing devices which reduce or wholly overcome some or all of the aforesaid difficulties inherent in prior known devices. Particular objects and advantages of the invention will be apparent to those skilled in the art, that is, those who are knowledgeable and experienced in this field of technology, in view of the following disclosure of the invention and detailed description of certain preferred embodiments.